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Paramatma BhattaraiMin Bahadur K.C.Telecom Engineer

Nepal TelecomTPID, DWT

DENSE WAVELENGTH DIVISION MULTIPLEXING



Agenda

Background of DWDM

DWDM Transmission Media

Key Technologies DWDM Network Design



Limitation of TDM

TDM Up gradation effects Service

Lack of rate upgrade flexibility

Higher Cost



Traditional TDM/SDM

RX

EDFA

TX

TX

TX

TX

TX

TX

TX

TX

RX

RX

RX

RX

RX

RX

RX

120 km 120 km 120 km

DWDM

EDFA

TX RXRegReg

TX RXRegReg

TX RXRegReg

TX RXRegReg

TX RXRegReg

TX RXRegReg

TX RXRegReg

TX RXRegReg

120 km 120 km 120 km

DWDM is the Main Expansion Scheme

Current DWDM Technology



Operation Modes

Two Fiber Bi-directional Transmission

Single Fiber Bi-directional Transmission

Optical Add Drop Multiplexing



Application Modes of DWDM

Open DWDM

Integrated DWDM



Advantages of DWDM

Ultra-large Capacity

Transparent Data rates

Cost Effect Up gradation

Flexible and Reliable Network

Compatible with AON



Future of DWDM

1996 2000 2004 2008

OADM

Point-point DWDM

Fixed wavelength

add/drop

OADM

OADM

OADM

OADM

Re-configurable

wavelength add/drop

OXC

OXC

OXC

OXC

OXC

Optical cross-connect

and mesh networking

More capacity,

less fibers

Flexible

bandwidth

assignment and

protection

schemes

Wavelengthconversion and

routing



Agenda

Background of DWDM





Optical Fiber Structure

Multi Mode Fiber (Step/Graded Index)

Core Diameter: 50/62.5 Micron

Light follows multiple paths in the fiber core Not Recommended for Long haul transmission

Single Mode Fiber (Step Index)

Core Diameter: 8~10 Micron

Light can follow only a single path in the fiber core

Suitable for long haul transmission



Fiber Characteristics affecting OSNR

Absorption Loss

Scatter Loss

Bend Loss

Fiber Loss

Fiber Dispersion

Chromatic Dispersion

Polarization Mode Dispersion

Fiber Non Linearity

Four Wave Mixing



Fiber Loss Spectrum and EDFA Gain Spectrum

EDFA Gain

Single-Mode Fiber

1.2 1.3 1.4 1.5 1.6 1.7Wavelength (m)

L o s s ( d B / k m

)

0.1

0.2

0.4

0.8

1.0

5000GHz



Fiber Characteristics: No linearity

Generating new frequencies with the interaction of the channels

Related parameters including the number of channels, the spacing

of channels and the power of channels

w1 w2

ww1 w22w1-w2 2w2-w1

w

Fiber

FWM Effect



ITU-T Defined Optical Fiber Types

G.652 Single Mode Fiber

G.653 Dispersion Shifted Fiber

G.654 Cut-off Wavelength Shifted Fiber

G.655 Nonzero Dispersion Shifted Fiber



G.652 Single Mode Fiber

also called Dispersion Unshifted Fiber

Extensively Used

Optimal for 1310nm Window

Based on refractive index cross-sectioncan be divided into two categories

a. Matched Cladding Fiber b. Depressed Cladding Fiber



G.653 Dispersion Shifted Fiber

1550nm Window Proprietary Optimal Fiber

Zero Dispersion shifted to 1550nm Window

Can implement Ultra high speed and Ultralong distance optical transmission

Not suitable for DWDM Application due to

FWM phenomena



G.654 Cut-off wavelength Shifted Fiber

Designed to reduce attenuation at 1550 nm

Zero Dispersion Point in 1310 nm window

Dispersion at 1550nm relatively high(i.e.18ps/nm/km)

Mainly used in submarine application



G.655 Nonzero Dispersion Shifted Fiber

Similar to G.653 fiber

Two Types

1. NZDSF+

2. NZDSF-

Preserves some Dispersion in 1550nm Window to

avoid FWM

Recommended for DWDM Application



Fiber Type and Loss

EDFA

bandwidth

1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (nm)

L o s s ( d B / k m )

0.1

0.2

0.4

0.81.0

0

-20

-10

10

20

D i

s p e r s i o n ( p s / n m - k m )

SMF

DSF

NZDF+

NZDF-

G.652

G.653

G.655+

G.655-

Dispersion/Loss for Various types of Fibers



Agenda

Background of DWDM


Key Technologies

DWDM Network Design



LASER

Regeneration Distance increased from 50~60to 500~600 Km.

Wavelength spacing from several

nanometer to sub-nanometer. Requirement for DWDM system.

a. Relatively large Dispersion tolerance

b. Standard and stable wavelength DWDM system uses Semiconductor LD



LASER Modulation Modes

Two Types

Direct Modulation Indirect Modulation



Direct Modulation (Internal Modulation)

Changing the laser intensity by controlling thedriving current

Extensively used in traditional PDH/SDH systems Simple Structure,Low Loss, Low Cost With passage of time it changes the length of laser

resonant cavity,hence varying the modulationcurrent will cause linear variation in emitting laserwavelength.This phenomena is called ModulationChirp.

Modulation Chirp is inevitable for Direct Modulation Broadens the emitting spectrum BW, deteriorates

spectrum characteristics, hence limiting transmissiondistance/rate



Indirect Modulation (External Modulation)

Constant

Light

Source

Optical

Modulator

Electric

Modulation signal

Output

Signal

Modulator acts as a Switch

Constant Laser is highly stable, is not effected by electric

modulation current during emission.

Spectrum characteristics of light wave will not be effected, hence

guaranteeing spectrum quality. High cost, but modulation chirp is very low

Can be classified into two categories

1. Integrated External Modulated Laser

2. Separated External Modulated Laser



Integrated External Modulated Laser

Becoming the developing trend for DWDM Lasers

Commonly used Modulator is EA Modulator

EA Modulator is compact and integrated with laser

1 o

Absorption

Region

2o

Absorption

Region

Biased

Unbiased

Can support transmission of 2.5Gb/s up to 600Km

1 = Absorption side wavelength of unbiased modulator

2 = Absorption side wavelength of biased modulator

0 = Operating wavelength of constant light source



Separated External Modulator Laser

Uses LiNbO3 MZ External Modulator

Separates light into two equal signals, entering two branches

Each branch uses electro-optic material,whose refractive

index changes with the magnitude of electrical modulationcurrent

Change in the refractive index of the optical branches willresult in variation of signal phase

When the signal at the output of two branches combine

together, the optical signal is an interfering signal withvarying phase characteristics

The frequency chirp of this type of modulation can be zero

Relatively lower cost compared to EA modulation



Wavelength Stability and Control

According to ITU-T G.692 deviation from centralwavelength should not be greater than one fifth (±1/10) ofoptical channel spacing,

i.e. wavelength deviation should not be greater than ±20

GHz for a system with channel spacing of 0.8nm 0.5 nm variation of wavelength can shift an optical channel

to another, critical for UDWDM

Practically the variation should be controlled within 0.2nm

Fine tuning for EML is implemented by adjusting

temperature Temperature sensitivity of wavelength is 0.08nm/C

DFB Laser is used to control the temperature of laser chipto achieve wavelength stability



Wavelength Stability and Control

Preferable to utilize wavelength sensitive componentfor feedback control of the Laser.

LASER Aging

LD Splitter

Wavelength

Sensitive

Component

Optical

Output

Signal

Processing

LD Control

Circuit

For Wavelength monitoring

For

Wavelength

Control

Standard wavelength control using feedback fromwavelength sensitive components is under research



EDFA Erbium Doped Fiber Amplifier

Key component of new generation OpticalCommunication Systems

Advantages High Gain

Large Output Power

Wide operating Bandwidth

Polarization independence

Amplification independent of bit rate and data format



Erbium DopedSilica Fiber

Isolator

Isolator Isolator

Pump Laser980 nm or 1480 nm

CouplerWDM

Pin Pout

EDFA Erbium Doped Fiber Amplifier



Principle of Light Amplification

980 nm pump

1480 nmpump

Fast non-radiation decay

E1

E3

E2

1530 - 1560 nmSimulated

Emission andSpontaneousEmission

1550 nmSignals

SimulatedAbsorption



Types of EDFA

Transmitter Receiver

Boost Amplifier Line Amplifier Pre Amplifier



Characteristics of Different types of EDFA

BoostAmplifier

LineAmplifier

Pre Amplifier

Gain Low High High

OutputPower

High Moderate/

HighLow

Noise Not Critical Low Noise Low Noise



Gain Control for EDFA

EDFA Gain Flatness EDFA based on Pure Silica has a very narrow flat gain

range 12 nm i.e. (1549~1561nm)

Gain fluctuation between 1530~1542 up to 8db

Solution: Use EDFA based on aluminum doped siliconFiber

EDFA Gain locking When channels are dropped, their energy is transferred to

un-dropped channels due to gain competition

Solution: Control gain of the pump through internalfeedback monitoring circuit



DWDM Components MUX/DEMUX

Should meet requirements defined by ITU-TG.671

Four types of widespread DWDM

Components

Diffraction Grating type WDM Component Filter type WDM Component

Coupler type WDM Component Integrated Wave guide type WDM

Component



Diffraction Grating Type WDM Component

Angular Dispersion Component

When light reaches the grating

surface, signals at differentwavelengths are reflected atdifferent angle, due to angulardispersion of grating

Diffraction grating type WDM

Signals then converged to different fibers via lenses

High resolution wavelength selection function can beimplemented using this technique

Extensively applied in DWDM systems



Filter Type WDM Component

Filters based on Dielectric Films

According to the Film interference

type, the Filter can be designed asBand pass for some wavelengths,and Band stop for otherwavelengths

Flat pass-band, polarization independence and structural

stability No. of wavelengths added or dropped cannot be large

Relatively longer design and manufacturing process andlow volume of production

Filter type WDM



Coupler Type WDM Component

A device capable of splittingwavelengths; two wavelengthson a single optical fiber into twofibers, or vice versa

Optical couplers are made byfusing/tapering two fiberstogether so that the cores areclose enough to each other forthe optical power to betransferred from one fiber to theother.

Extensively applied in DWDM and EDFA applications

Coupler type

1

2

3

4

5

6

7

8



Integrated Wave guide type WDM Component

A plane wave guide component based on optical integrationtechnology

Can be fabricated into matrixstructure to add/drop opticalchannels (used in OADM)

Typical wave guide type is AWG type manufactured by NTT

Advantages: Small wavelength spacing, large no. of channels,flat pass band

Suitable for ultrahigh speed/capacity DWDM

AWG type WDM



Agenda

Background of DWDM





Network Element types of DWDM

OTM Optical Terminal Multiplexer

OLA Optical Line Amplifier

OADM Optical Add Drop Multiplexer

REG Electrical Regenerator



OTM Optical Terminal Multiplexer

Transmitter End Multiplex STM-N signals to M wavelengths 1 to M

Amplify Optical power

Add Optical Supervisory Channel s (generally 1510 nm)

Receiver End Extract and process OSC

Amplify Optical channels and Demultiplex into M STM-N signals



OLA Optical Line Amplifier

Extract and process OSC

Amplify the main Optical channel

Multiplex OSC back to the Optical channel



OADM Optical Add Drop Multiplexer

Static/Fixed OADM

Can be implemented via a single board/unit

Board/Unit capable of wavelength conversion

Capable of adding/dropping 1~8 wavelengths Back to back OTM

More flexible compared to fixed OADM

Can Add/Drop all M wavelengths at certain node



REG Electrical Regeneration Unit

No capability to Add/Drop service

Used to elongate Dispersion limited distance



DWDM Network Design

Point-to-point Network

Chain Network

Ring Network



Point to Point Topology

SDH OTM OLA SDH OTM



Chain Topology

SDH OTM

SDH OTM

OADM SDH



Ring Topology

OADM

OADM

OADMOADM

1~8

1~8

1~8

1~8



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